ABSTRACT The anti-nuclear autoantibodies (ANA) present in lupus are high-affinity, class-switched Abs that arise through T cell-dependent germinal center (GC) reactions. GC development and persistence is dependent on help from Follicular Helper CD4+ T cells (TFH). Although aberrant development and function of TFH drive disease in both SLE patients and murine models of lupus, the molecular and cellular pathways that initiate and reinforce the TFH fate remain poorly defined. This limits our ability to dissect the altered biology underlying lupus. Additionally, the study of human TFH has been hampered by lack of access to the secondary lymphoid tissues where they differentiate and function. Changes in the epigenetic code mediate the identity, specificity, stability, and function of differentiated CD4+ T helper (Th) cell subsets. Indeed, the epigenetic landscape may be a better indicator of the developmental and functional relationships amongst Th subsets than lineage-specific transcription factors. In preliminary data, our epigenetic analyses show that a significant number of lupus susceptibility GWAS SNPs map to regions of open chromatin in TFH, suggesting that altered TFH biology mediates a significant component of lupus genetic risk. We previously used unique murine models to define discreet stages of TFH differentiation: conventional DCs prime CD4+ T cells toward an unstable pre-TFH intermediate that expresses Bcl6 and CXCR5. Cognate T-B interactions drive the pre-TFH to produce IL-21, express PD-1, and repress production of IL-17. Preliminary epigenetic analyses confirm that pre-TFH are not fully committed to the TFH fate and show that we can identify a B cell-dependent module. However, B cell MHCII expression--with no requirement for DC priming--is sufficient for TFH differentiation in immunological settings similar to lupus. The proposal will utilize unique murine models and primary human cells to establish the epigenetic roadmap that defines commitment to the TFH fate. Lupus-prone mice with limited expression of MHCII will be utilized to define how sequential interactions between CD4+ T cells and DCs followed by B cells mediate changes in the epigenetic code. ATAC-Seq analyses of pre-TFH and TFH will identify the regulatory cascades, transcription factor programs, and metabolic pathways that are regulated by DCs and B cells during TFH differentiation. Secondly, we will build on these analyses to define how lupus disrupts this transcriptional and functional program. Does lupus: 1. alter the requirement for sequential cDC and B cell antigen presentation in the differentiation and maintenance of TFH? or 2. disrupt the epigenetic landscape of TFH differentiation and commitment? These experiments should identify the epigenetic signatures that 1. Define the B cell contribution to TFH differentiation and 2. Distinguish TFH in lupus and health controls. These results will provide the framework for future therapeutic interventions to ameliorate disease.